In many filtration applications, a filtration device is comprised of both a filter element, such as a septum, and a filter-aid material. The filter element may be of any form such that it may support a filter-aid material, for example, a cylindrical tube or wafer-like structure covered with a plastic or metal fabric of sufficiently fine weave. The filter element may be a porous structure with a filter element void to allow material of a certain size to pass through the filtration device. The filter-aid material may comprise one or more filtration components, which, for example, may be inorganic powders or organic fibrous materials. Such a filter-aid material may be used in combination with a filter element to enhance filtration performance. Often, filtration components for use in a filter-aid material are comprised of materials such as diatomite, perlite, and cellulose. As an example illustrative of the field of filtration, the filter-aid material may initially be applied to the septum in a process known as “pre-coating.” Pre-coating generally involves mixing a slurry of water and filter-aid material and introducing the slurry into a stream flowing through the septum. During this process, a thin layer, such as about 1.5 mm to about 3.0 mm, of filter-aid material may be eventually be deposited upon the septum, thus forming the filtration device.
During the filtration of a fluid, various insoluble particles in the fluid are trapped by the filter-aid material. The combined layers of filter-aid material and particles and/or constituents to be removed accumulate on the surface of the septum. Those combined layers are known as “filter cake.” As more and more particles and/or constituents are deposited on the filter cake, the filter cake may become saturated with debris to the point where fluid is no longer able to pass through the septum. To combat that problem, a process known as “body feeding” is often used. Body feeding is the process of introducing additional filter-aid material into the fluid to be filtered before the fluid reaches the filter cake. The filter-aid material will follow the path of the unfiltered fluid and will eventually reach the filter cake. Upon reaching the filter cake, the added filter-aid material will bind to the cake much the same way the filter-aid material bound to the septum during the pre-coating process. That additional layer of filter-aid material causes the filter cake to swell and thicken and increases the capacity of the cake to entrap additional debris. The filter aid typically has an open porous structure which maintains an open structure in the filter cake, thus ensuring continued permeability of the filter cake.
As mentioned above, in the field of fluid filtration many methods of particle separation employ, for example, materials chosen from diatomite, expanded perlite, natural glasses, and cellulose materials as porous filtration components. Those materials have intricate and porous structures that may be particularly suited to the effective physical entrapment of particles in filtration processes. Those intricate and porous structures create networks of void spaces that may result in buoyant filtration media particles that have apparent densities similar to those of the fluids in which they are suspended. It is common practice to employ porous filtration components when improving the clarity of fluids. The porous filtration component is often used to remove undesired particles or constituents such as particulate matter from a fluid. However, while well suited for the task of removing particulate matter by physical entrapment, those porous filtration components may not be as well suited for the task of removing particulate matter from a fluid by the process of adsorption and are thus often times used in combination with an adsorbent component.
Diatomite, perlite, rice-hull ash, and cellulose are some examples of filtration component materials that may be used for particle separation. Diatomite, also known as diatomaceous earth, is a sediment enriched in biogenic silica in the form of the siliceous frustules of diatoms, a diverse array of microscopic, single-cell algae. Those frustules are sufficiently durable to retain much of their microscopic structure through long periods of geological time and through thermal processing. Diatomite products have an inherently intricate and porous structure composed primarily of silica. Perlite is a naturally occurring volcanic glass that may thermally expand upon processing. The structure of perlite may not be as intricate as diatomite and, consequently, perlite may be better suited for separating coarse micro-particulates from liquids having high solids loading. Finally, cellulose filtration component materials are generally produced by sulfite or sulfate processing of hardwoods and/or softwoods. Like perlite, cellulose filtration component materials may possess a less intricate structure than diatomite filtration component materials.
As used herein, “turbidity” is the cloudiness or haziness of a fluid, where the haze may be caused by individual particles that are suspended in the fluid. Materials that may cause a fluid to be turbid include, for example, clay, silt, organic matter, inorganic matter, and microscopic organisms. Turbidity may be measured by using an instrument known as a nephelometric turbidimeter that emits a beam of light through a column of the fluid being tested. A detector positioned on the same side of the fluid column measures the amount of light reflected by the fluid. A fluid that contains a relatively large number of suspended particles will reflect a greater amount of light than a fluid containing fewer particles. Turbidity measured in this fashion may be quantified in Nephelometric Turbidity Units (“NTU”). Turbidity may also be measured via gravimetric methods.
A trade-off typically exists in filter-aid technology between the permeability of the porous media used as a filtration component and its turbidity removal capabilities. Filtration components are produced in grades over a wide range of permeability ratings. As the permeability of the filtration component decreases, the ability of the filter-aid material to remove small particles may increase, but often at the expense of a slower flow rate through the filter-aid material. Conversely, as the filtration component permeability increases, the ability of the filter-aid material to filter particles may decrease and, consequently, the fluid flow through the filter-aid material increases. The extent to which this takes place will depend upon the type and particle size distribution of the suspended particles being removed from the fluid.
As used herein, “wet density” is an indicator of a material's porosity. For example, wet density reflects the void volume available to entrap particulate matter in a filtration process and, consequently, wet density may be used to determine filtration efficiency. Percent porosity may be expressed by the following formula:Porosity=100*[1−(wet density/true density)].
Thus, filtration components with lower wet densities may result in products with greater porosity, and thus perhaps greater filtration efficiency, provided that the true density stays relatively constant. Typical wet densities for common filtration components may range from at least about 12 lb/ft3 to about 30 lb/ft3 or greater.
As used herein, “adsorption” is the tendency of molecules from an ambient fluid phase to adhere to the surface of a solid. This is not to be confused with the term “absorption,” which results when molecules from an ambient fluid diffuse into a solid, as opposed to adhering to the surface of the solid.
To achieve a desired adsorptive capacity, and thus to be practical for commercial use, an adsorbent component may have a relatively large surface area, which may imply a fine porous structure. In certain embodiments, porous adsorbent components, in their un-reacted powder form, can have surface areas ranging up to several hundred m2/g.
One technique for calculating specific surface area of physical adsorption molecules is with the Brunauer, Emmett, and Teller (“BET”) theory. The application of BET theory to a particular adsorbent component yields a measure of the materials specific surface area, known as “BET surface area.” Generally speaking, BET surface areas of practical adsorbent components in their un-reacted powder form may range from about 50 to about 1200 m2/g. As used herein, “surface area” refers to BET surface area.
Filtration components with different BET surface areas and/or different total pore areas may result in different adsorption capacity and filtration rate. Typically, a filter aid with a lower BET and/or lower total pore area tends to have a lower adsorption capacity and a faster filtration rate. Calcined diatomaceous earth filter aids and expanded and milled perlite filter aids are generally used as filter aids with minimal adsorption function, because of the low surface area, typically<10 m2/g. Adsorbent components, such as silica gels, are generally high in BET surface areas or total pore areas, but their filtration rates are generally slow, due to a much finer particle size distribution and/or the lack of the porosity of a filter aids. The fine particles can block the pores in filtration, and the high surface area may create more drag on the flow, thus causing the filtration rate drop significantly.
One technique for describing pores size distributions uses mercury intrusion under applied isostatic pressure. In this method an evacuated powder is surrounded by liquid mercury in a closed vessel and the pressure is gradually increased. At low pressures, the mercury will not intrude into the powder sample due to the high surface tension of liquid mercury. As the pressure is increased, the mercury is forced into the sample, but will first intrude into the largest spaces, where the curvature of the mercury surface will be the lowest. As pressure is further increased, the mercury is forced to intrude into tighter spaces. Eventually all the voids will be filled with mercury. The plot of total void volume vs. pressure can thus be developed. The method can thus provide not only total pore volume but also distinguish a distribution of pore sizes. Note that Mercury Intrusion Porosimetry cannot distinguish between intra- and inter-particle voidage and thus some knowledge of particle size and shape may be needed for plot interpretation. Furthermore, some pore shapes (such as large pores with small access ports, the so-called inkwell pore) can fill at misleadingly high pressures, so in effect the method is providing an estimation of the true pore size distribution and not a direct measurement. Once a distribution of pores has been estimated, it is possible to calculate an estimation of surface area based on the pore sizes, assuming a pore shape (a spherical shape is commonly assumed). Median pore size estimates can also be calculated based on volume or area. Median pore size (volume) is the pore size at 50th percentile at the cumulative volume graph, while median pore size (area) is the 50th percentile at the cumulative area graph. The average pore size (diameter) is 4 times the ratio of total pore volume to total pore area (4V/A).
One method of using an adsorbent component is to place the adsorbent component in contact with a fluid containing particles and/or constituents to be adsorbed, either to purify the fluid by removing the particles and/or constituents, or to isolate the particles and/or constituents so as to purify them. In certain embodiments, the adsorbent component containing the adsorbed particles or constituents is then separated from the fluid, for example by a conventional filtration process.
An illustrative example of an adsorption practice may be seen in the process of beer “chill-proofing.” It is currently known that, unless specially treated, chilled beer may undergo a chemical reaction that results in the production of insoluble particles. In that chemical reaction, hydrogen bonds may form between haze-active proteins and/or polyphenols in a chilled condition. The reacted proteins and/or polyphenols may then grow to large particles, which cause the beer to become turbid, a condition also known as “chill-haze.” Chill-haze may be undesirable to both consumers and brewers. Turbidity may be most pronounced when the beer has been chilled below room temperature. In certain instances, such as when the particles are proteins, as the temperature increases, the hydrogen bonds that hold the proteins together may be broken.
Chill-proofing may comprise a process that employs at least one adsorption component and/or at least one filtration component to remove particles creating chill-haze in the beer. One form of chill-proofing involves, in one step, adding solid adsorbent components, such as silica gel, to the beer prior to packaging. The particles and/or constituents bind to the added adsorbent components, and then, in a second step, the adsorbent components are subsequently filtered from the beer, which is then packaged for storage, sale, and/or consumption.
Filtration processes that implement both an adsorption step and a filtration step may be less efficient because of the difficulties of filtering the adsorbent components. For example, the adsorbent components may occupy void spaces of the porous filter-aid material. That occupancy may reduce the permeability of the filter-aid material, leading to an overall lower filtration flow rate, or may require the addition or more filter-aid material at additional cost, and may also result in the faster consumption of available volume in the filter housing.
There have been previous attempts to improve upon the traditional process of chill-proofing. Earlier attempts involved creating a simple mixture of an adsorbent component and a filtration component to combine the filtration and adsorption steps into one, thus eliminating the need to filter the adsorbent components. The term “simple mixture” is used herein to describe a composition comprising at least one adsorbent component and at least one filtration component where the two components are not chemically bonded, thermally sintered, or precipitated together. Simple mixtures may be somewhat ineffective as the components may be subject to separation due to physical distress often experienced in packaging and shipping. Furthermore, the particle shape characteristics of the adsorbent component may mean that these particles do not aid filtration in the way the filtration component particles do by ensuring the continued permeability of a filter cake. Thus the particles of the adsorbing component would take up valuable void space in the filter cake, thus reducing permeability or requiring more of the filtration component to maintain permeability.